The present invention relates to methods of forming a graphene layer on commercially available large substrates, and structures obtained by the same.
Graphene is a structure consisting of carbon as a two-dimensional sheet. A graphene monolayer has a thickness of about 0.34 nm, i.e., which is approximately the van der Waals atomic diameter of a single carbon atom. As defined by IUPAC, graphene is a one-atom thick, two-dimensional (2D) sheet of C atoms arranged in a hexagonal configuration. As used herein, a graphene layer can be a single two-dimensional sheet (one monolayer) of graphene, or alternately, a graphene layer can be a stack of a plurality of two-dimensional monolayers of graphene, which do not exceed more than 10 monolayers and is typically limited to less than 5 monolayers. Graphene provides excellent in-plane carrier mobility. Semiconductor devices employing graphene have been suggested in the art to provide high-switching-speed semiconductor circuits. Carbon atoms are arranged in a two-dimensional honeycomb crystal lattice in which each carbon-carbon bond has a length of about 0.142 nm.
A graphene layer may be grown by direct epitaxial deposition of carbon atoms on, i.e., addition of carbon atoms onto the surface of, a surface of a single crystalline silicon carbide (SiC) substrate having a hexagonal symmetry such as a (0001) surface of alpha silicon carbide. Alternately, graphene can be grown by heating a hexagonal surface of a single-crystalline silicon carbide material at a temperature greater than 1,100° C. The process of forming graphene by such a high temperature anneal is subtractive epitaxy, i.e. it is actually a reduction process in which silicon atoms on a hexagonal surface of a silicon carbide crystal sublime during the anneal and the remaining carbon atoms reassemble to form graphene that usually has an epitaxial relation to the substrate crystal.
Alpha silicon carbide has a hexagonal crystal structure, and beta silicon carbide has a cubic crystal structure of zinc blende type. FIG. 1 schematically shows the crystallographic structure of alpha silicon carbide. A (0001) surface is perpendicular to the c-axis and atoms in the (0001) surface are arranged in a pattern having a hexagonal symmetry. The plane in which the (0001) surface is located is referred to as a C plane. A (1102) surface of alpha silicon carbide crystal has a cubic symmetry, and does not have a hexagonal symmetry.
Silicon carbide substrates having a (0001) surface orientation are not commercially available at a diameter greater than 4 (or 5) inches at the present time. Such unavailability of silicon carbide (SiC) substrates currently makes it impossible to provide a 200 mm substrate or a 300 mm substrate containing a graphene layer. Thus, formation of graphene by epitaxy on a hexagonal surface of a silicon carbide crystal is limited to epitaxial deposition process performed directly on commercially available silicon carbide substrates having a hexagonal symmetry, but cannot be performed on substrates having a diameter of 6 inches or greater.
Prior art methods for forming a graphene layer on a single crystalline silicon carbide (SiC) substrate either by deposition or by subtraction of silicon has required a surface having a hexagonal lattice symmetry, which is the same type of lattice symmetry as the in-plane symmetry of the graphene layer. In other words, formation of a graphene layer on a silicon carbide single crystal as known in the art has required a (0001) surface of alpha silicon carbide having a hexagonal crystal structure. The (0001) surface of alpha silicon carbide has a hexagonal symmetry. Growing high quality graphene on SiC grown on Si by subtractive epitaxy is complicated, due to the fact that the Si substrate melts around 1400° C., and high quality graphene is preferably formed at a temperature greater than 1400° C. Furthermore, even if graphene is formed below 1400° C., there will be a quite substantial Si vapor pressure in the vicinity of SiC, due to the presence of the Si substrate, which may suppress the sublimation of Si from SiC needed to form graphene (see for example Tromp and Hannon Phys. Rev. Lett. 102, 106104 (2009)). Thus working with a substrate that does not contain Si, is very advantageous for the formation of graphene at T>1400° C.